In the mid-20th century polio ravaged the United States, causing paralysis in more than 15,000 people each year. Vaccines created in the 1950s and ’60s have helped bring the disease to the point where no documented cases have originated in this country since 1979.
The U.S. and most of the world have what’s called herd immunity to polio, a level of protection that keeps the virus from transmitting. Though vaccines are still needed because the poliovirus circulates in some Asian and African countries, Americans are not at risk of contracting the disease domestically.
The concept of herd immunity has been raised a lot regarding the coronavirus. It has been held out as the eventual goal of the vaccination campaign sweeping the country, but true elimination of the disease may not be possible.
“As we see how it’s playing out, COVID-19 will become endemic,” Teton County Director of Health Jodie Pond said. “It is not going to be a polio deal where we are able to eradicate it from the Earth.”
Herd immunity thresholds vary by disease and are generally higher for ones that are more transmissible. Immunologists estimate that the percentage is around 75% to 80% for diphtheria, 80% for polio and 95% for measles.
What that number will be for COVID-19 isn’t yet clear. Most estimates are in the 70% to 90% range, but because no community has yet reached complete protection and the virus is so widespread, it is not yet possible to determine the threshold.
The Teton County Health Department, Pond said, is operating with an estimate of 80% for herd immunity, though officials hope for an even higher level of protection. At the state level, Wyoming Department of Health spokeswoman Kim Deti said the state isn’t using an estimate.
“We are focused on encouraging as many people as possible to say yes to these vaccines and making it as easy as possible for them to get vaccinated,” Deti wrote in an email. “The more people who are vaccinated, the more people will have protection against getting seriously ill from COVID-19, and that is our goal.”
In its truest sense, herd immunity includes people who have had a natural infection from a disease. Adding the number of natural infections to the number of people vaccinated gives the percentage of people protected from the disease.
However, most health officials are not considering natural infections when calculating community protection from COVID-19, instead only using the percentage of people who have been fully vaccinated. That’s because COVID-19 is so new that it’s unclear how long antibodies last, how many are produced from a natural response and what level is required for protection.
“Most people who are infected with COVID-19 develop an immune response within the first few weeks, but we don’t know how strong or lasting that immune response is, or how it differs for different people,” the World Health Organization website says.
Scientists are studying how many antibodies people produce both from natural infections and vaccines, but it will take more time for them to finalize such results. In the meantime, public health officials are simply working to protect as many people as possible.
“There has been a particular focus on getting those who are more vulnerable vaccinated,” Deti said. “Our goal is to prevent serious illness in as many individuals as possible.”
It’s also still unclear how long protection lasts for those who have been vaccinated. Drugmakers are continuing to track the effectiveness of vaccines, but Pfizer and Moderna have recently said that antibody levels remain high for at least six months after vaccination. That number could increase, but that’s how long they have studied the levels in vaccinated people.
With a global disease like COVID-19, herd immunity isn’t necessarily protective if just Teton County reaches it. If Wyoming and the United States also don’t reach high levels of protection, domestic tourism will likely circulate the disease back into Teton County, risking infection for unvaccinated people and raising the likelihood of breakthrough cases in those who are immunized.
“We could be at 80% in Teton County,” Pond said. “But if the rest of the state and the rest of the nation and the rest of the world isn’t at high vaccination rates, nobody’s safe until we’re all safe.”